This invention relates to a dot-matrix impact printer for printing characters, symbols, and other information on media such as paper by means of wire dot impact.
Dot-matrix impact printers are widely used as output devices of information-processing apparatus such as personal computers. A prior-art dot-matrix impact printer is shown in block diagram form in FIG. 1. Data from the information-processing apparatus are received via an interface circuit 100 and applied to a central processing unit (hereinafter referred to as a CPU) 101 which controls the operation of the printer. The CPU 101 communicates with other parts of the printer via an integrated I/O circuit (an I/O circuit formed of a large-scale integrated circuit) 102 which transfers signals from the printer's control panel 106 to the CPU 101 and transfers signals from the CPU 101 to a timer circuit 103, a drive circuit 104, a line-feed motor 107, and a spacing motor 108. The drive circuit 104 drives wires in a wire-dot print head 105, causing the printing of characters or other information.
The control panel 106 comprises, for example, one or more pressure-sensitive membrane switches (not shown in the drawing) which, when pressed, generate electrical signals that are sent via the I/O circuit 102 to the CPU 101. The CPU 101 responds to these signals and to data received via the interface circuit 100 by controlling the timer circuit 103, the drive circuit 104, the line-feed motor 107, and the spacing motor 108 so that the desired information is printed by the wire-dot print head 105. The line-feed motor 107 moves the paper in the vertical direction and the spacing motor 108 moves the wire-dot print head 105 in the horizontal direction, enabling characters to be printed at different positions.
FIG. 2 is a schematic diagram showing an example of part of the timer circuit 103 in FIG. 1, associated with one print wire. As illustrated, it comprises an open-collector NOT gate 109, a comparator 110, resistors 111, 112, and 113, a diode 114, and a capacitor 115. This circuit receives an input timing signal t.sub.1 from the I/O circuit 102 and generates an output timing signal t.sub.2 which it sends to the drive circuit 104 in FIG. 1.
FIG. 3 is a timing chart illustrating the operation of the timer circuit in FIG. 2. The signal t.sub.1 received from the I/O circuit 102, which is a pulse signal with a High duration of T.sub.1 as shown in at (a) in FIG. 3, is inverted by the NOT gate 109, so when the signal t.sub.1 goes High, the output signal of the NOT gate 109 goes Low, allowing the capacitor 115 to discharge to ground level. At the end of time T.sub.1 the input of the NOT gate 109 goes Low again and its output returns to the High level (open state), causing the voltage Vh to charge the capacitor 115 through the resistor 111 with an RC time constant determined by the resistance (R111) of the resistor 111 and the capacitance (C115) of the capacitor 115. The output voltage of the NOT gate 109 rises as the capacitor 115 charges, as indicated in at (b) in FIG. 3. This rising voltage is received at the invert input terminal of the comparator 110. The comparator 110 receives at its non-invert input terminal a reference voltage determined by the resistance R112 of the resistor 112 and the resistance R113 of the resistor 113, according to the formula: EQU Reference voltage=R113.multidot.Vcc/(R112+R113)
The output t.sub.2 of the comparator 110 thus remains at the High level for the time T.sub.2 until the charge in the capacitor 115 reaches the reference voltage level, as shown in at (c) in FIG. 3. The output signal t.sub.2 thus generated by the timer circuit 103 is referred to as the Overdrive signal.
By circuits similar to the circuits 109 to 115, the timer circuit 103 also generates an output signal t.sub.3 which goes High together with t.sub.1 and remains High for a longer time T.sub.3 (where T.sub.3 &gt;T.sub.2). The signal t.sub.3 is referred to as the Enable signal. Identical circuits generate separate Overdrive and Enable signals and send them to the drive circuit 104. The drive circuit 104 also receives Print signals t.sub.4 from the I/O circuit 102.
A part of the drive circuit 104 associated with one print wire is shown in FIG. 4. As illustrated, it comprises a buffer amplifier 116, an AND gate 117, NPN transistors 118 and 120, a PNP transistor 119, diodes 121 and 122, and resistors 124 and 125, which are connected to a head coil 123 for driving an associated print wire. The Overdrive signal t.sub.2 is received by the buffer amplifier 116, while the Enable signal t.sub.3 and Print signal t.sub.4 are received by the AND gate 117. The timing of these inputs is shown in FIG. 5. The Print signals select the wire to be driven. When the wire-dot print head 105 is at a given position on the paper, Print pulses are supplied only for the wires to be driven at that position.
When the illustrated part of the drive circuit 104 receives an Overdrive signal t.sub.2, the NPN transistor 118 and the PNP transistor 119 both turn on. When the drive circuit 104 receives both an Enable signal t.sub.3 and a Print signal t.sub.4, the output of its AND gate 117 goes High, turning on the NPN transistor 120. A drive current I.sub.H is then permitted to flow from the power supply, which provides a voltage Vh, on a path marked R.sub.1 in FIG. 4 through the PNP transistor 119, the head coil 123, and the NPN transistor 120 to ground. This current flows during the interval d.sub.1 in at (d) in FIG. 5.
When the Overdrive signal t.sub.2 goes Low, the NPN transistor 118 and the PNP transistor 119 both turn off, but the electromotive force generated by the head coil 123 causes a residual current to flow on the path marked R.sub.2, circulating from the head coil 123 through the NPN transistor 120 and the diode 122, then back to the head coil 123. The current I.sub.H flowing through the head coil 123 therefore decreases gradually during the interval d.sub.2 in at (d) in FIG. 5.
When the Enable signal t.sub.3 goes Low, the output of the AND gate 117 also goes Low, turning off the NPN transistor 120 and changing the current path to the path marked R.sub.3 in FIG. 4, from ground through the diode 122, the head coil 123, and the diode 121 to the power supply. The current I.sub.H flowing through the head coil 123 therefore rapidly decreases as indicated in the interval d.sub.3 in at (d) in FIG. 5.
The way in which the current flowing through the head coil 123 drives the print wire will be explained next.
FIG. 6 shows a sectional view of the part of the wire-dot print head 105 for driving a print wire 131. For the purpose of explanation of the print head, the direction toward a printing paper PM in which the print wires are driven, i.e., the upward direction as seen in FIG. 6 is referred to the forward direction or front. The head coil 123 is wound around a core 135 to form an electromagnet. The core 135 is secured to a base plate or rear yoke 137, at the perimeter of which is fastened a permanent magnet 138. Mounted on the permanent magnet 138 in sequence from bottom to top in FIG. 6 are an upright support 139, a spacer 140, a plate spring 134, a front yoke 141, and a guide frame 130, the entire assembly being secured by an external clamp 142. An armature 132 is fastened to the inner free end of a radial part 134a of the plate spring 134, and the armature 132 is mounted on the plate spring 134. A print wire 131 is mounted to the armature 132. The tip of the print wire 131 extends through a central hole or a guide aperture in a guide frame 130 forward (upward in the drawing), i.e., toward the printing paper PM on the platen PL and out of the guide frame 130.
A magnetic flux circuit is formed from the permanent magnet 138, through the core 135, the armature 132, and the front yoke 141 back to the permanent magnet 138. When the head coil 123 is not energized, the flux generated by the permanent magnet 138 acts through the core 135 to attract the armature 132, thereby resiliently deforming the plate spring 134 as shown in FIG. 6, causing the print wire 131 to be kept retracting in the guide frame 130. When the head coil 123 is energized, it creates a flux in the core 135 that acts counter to the flux generated by the permanent magnet 138, thus weakening the attractive power of the core 135, allowing the plate spring 134 to recover by the force of its own resiliency and drive the print wire 131 upward in FIG. 6. The end of the print wire 131 then presses an ink ribbon IR against the printing paper PM on the platen PL to print a dot.
The print wires 131 in the wire-dot print head 105 are driven as selected by the Print signals as the wire-dot print head 105 moves back and forth and the paper moves in the feed-direction to print characters, symbols, and other information on the paper.
When the head coil 123 is de-energized, the flux from the permanent magnet 138 is reasserted in the core 135 and again attracts the armature 132 to the core 135, thus retracting the print wire 131.
The optimum energization time (drive time) of the head coil 123 varies depending on the printing conditions, including such factors as the time taken by the tip of the print wire 131 to reach the paper, the magnitude of the voltage Vh applied to the head coil 123, the number of print wires to be driven simultaneously, and the distance from the tip of the print wire to the paper (called the head gap). The pulse width T.sub.1 of the signal t.sub.1 is determined by the CPU 101 according to the number of wires to be driven simultaneously. As explained above, this time T.sub.1 is extended in the timer circuit 103 to the time T.sub.2, the amount of the extension being the time taken for the capacitor 115 to be charged through the resistor 111 by the voltage Vh, the extension thus being shorter when Vh is large and longer when Vh is small. The Overdrive signal t.sub.2 is thus corrected not only for the number of print wires driven simultaneously, but also for variations in the voltage Vh applied to the head coil 123.
Although this system is capable of optimizing the drive time with respect to the two factors just mentioned, it does not enable the printing force (the force of impact of the print wires on the paper) to be varied freely in response to such factors as the thickness of the paper or the number of copies printed simultaneously. Yet different types of paper and types of printing have different optimum impact forces. Thin paper, for example, does not require a large impact force, and a small impact force is preferable in that it reduces the noise of the printing process.
If, however, the impact force is reduced by shortening the drive time of the head coil 123, the impact force may become unstable, degrading the quality of the printing. Due to unavoidable manufacturing variations in the wire-dot print heads, some print wires may fail to print at all.
Another problem is that if the impact force is adjusted to the optimum value for thin paper, when thick paper is used the impact force will be inadequate and the printing will be faint.
For this reason, in the prior art the impact force of the printer is adjusted for thick paper, causing a strong force to be employed even when it is not needed. This results not only in unnecessary noise, but also in unwanted indentations of the paper where the dots are printed.